1. Field of the Invention
This invention relates generally to the processing of industrial process gases and specifically to the cooling of raw coke oven gas as the initial step in the processing of the by-product gases therein.
2. Description of the Prior Art
The cooling of raw coke oven gas was traditionally done by air, a method of cooling well known and widely used in the past in the field of art. Initially, the coolers were comprised of a plurality of large diameter upright pipes wherein the raw coke oven gas was conducted through the interior of these pipes and the natural convection of air currents cooled the outside of the pipes, transferring the heat from the raw coke oven gas to the metal pipe and then to the surrounding air where that heat was dissipated. Various improvements were attempted with such coolers including the use of forced draft convection and air flow regulators. However, basic problems which turned out to be commercially insurmountable were found to be inherent in the air-cooling method. Cooling rates were too slow and cooling surface requirements were too great. Also, on particularly hot summer days the direct sun would heat the air coolers to a degree higher than the desired discharge temperature.
Thus the use of air coolers was eventually abandoned in favor of liquid cooling systems. The early liquid cooling systems comprised a primary cooler in the form of a packed column, 50 to 80 feet high. Flushing liquor was pumped to the top of the tower and trickled down over an arrangement of hurdles made of wood slats. The raw coke oven gas was introduced at the bottom of the tower from where it rose upwardly through the hurdles and across the wood slats, making intimate contact with the flushing liquor filtering downwardly. The hot flushing liquor was then pumped from the bottom of the packed column to a cooling and recycling system. Inherent in this system was a loss of static pressure in the gas system. This pressure differential was further increased by the buildup of naphthalene crystals on the slats, insoluble in the water which comprised the aqueous base for the flushing liquor.
The current practice, developed to lower the gas pressure differential, is a water spray system which provides finely divided spray droplets to form a heat transfer surface. Again, a column or tower is used, but not packed. A three-phase spray system is arranged in the tower, including a primary spray section, a secondary or respray section, and a pumping section. Cold water enters the sprays at the top of the tower where there are sufficient spray heads to ensure that the entire volume of the primary spray section is filled with uniformly distributed droplets. After contacting the rising gas, the liquid collects on the top of a gas-distributor plate located at the base of the primary spray section. Most of the water then flows to a respray pump and is recycled to the secondary or respray section where it contacts the hot gas entering the lower section of the tower. The water then collects at the pumping section at the base of the tower where the tar extracted from the raw coke oven gas is allowed to settle and time is allowed for entrained gas bubbles to escape. The hot water is then pumped to a cooling system separate from the tower. The cooling system usually takes the form of an evaporation tower, or a more complex system of heat exchanger towers. Water from the cooling system is then recirculated.
The water spray system described above is classified as a "direct" primary cooler. "Indirect" primary coolers are also well known and currently being used as an alternative to the direct system. Indirect coolers are large box-shaped shell-and-tube heat exchangers in which raw coke oven gas flows through the shell and passes countercurrently to cooling water flowing through tubes. In such a system, the water never actually comes in contact with the gas but, instead, cools the tubes which, in turn, cool the gas flowing around them. The hot water exiting from the indirect cooler is, like the direct cooler system, cooled and recycled.
The water-cooling systems utilized in conjunction with both the direct and indirect primary coolers produce certain operational aspects which are objectionable in the environmental protection sense. Pollution laden steam from the cooling tower discharges directly into the atmosphere as well as promotes the formation of ice on streets and other passageways during winter by condensation of that steam. Noise, at an objectionable decible level, is produced from the combination of convection fans, rushing water, and water flashing into steam. Relatively large quantities of water are required, requiring costly large diameter pipelines, to replace water lost by evaporation and over spray. The water supply in many cases must be filtered and additives introduced to protect against corrosion as well as mineral and scum buildup and accumulation. In some systems there is also an economic necessity to discharge excessively warm water into rivers, creating a thermal pollution problem similar to that experienced with nuclear reactors.
The present invention provides a method and apparatus by which the disadvantages inherent in the previously described primary cooling processes, both air-cooling and water, are largely avoided.